1. Field of the Invention
The present invention generally relates to an automatic transmission control apparatus and method of controlling the automatic transmission. More specifically, the present invention relates to an automatic transmission control apparatus with a plurality of frictional engaging elements that are selectively engaged to achieve a gear, and configured to accomplish upshifting of the automatic transmission when the vehicle is in a power-off running state by interchanging between engagement of a first frictional engaging element serving to achieve a low gear position before shifting and engagement of a second frictional engaging element serving to achieve a high gear position after shifting.
2. Background Information
When an automatic transmission shifts gears (changes gear ratios), a clutch or other frictional engaging element is typically changed from a released state to an engaged state or from an engaged state to a released state. It is preferable to operate the frictional engaging element smoothly and quickly such that shock does not occur due to shifting gears. Various technologies have been developed for this purpose. Examples of such automatic transmissions are disclosed in U.S. Pat. No. 5,782,711 (also published as Japanese Laid-Open Patent Publication No. 09-17065) and Japanese Laid-Open Patent Publication No. 2000-110929.
The technology disclosed in U.S. Pat. No. 5,782,711 is a technology for reducing the shock associated with shifting gears by controlling a hydraulic pressure supplied to a hydraulic servo mechanism of a frictional engaging element. As shown in FIG. 3 of U.S. Pat. No. 5,782,711, a frictional engaging element is changed from a released state to an engaged state (such a frictional engaging element operation is called “engaging frictional engaging element” or “closing frictional engaging element”), a target hydraulic pressure PTA is calculated for starting an inertia phase based on an input torque, and a prescribed slope (rate of change) is calculated based on the target hydraulic pressure PTA and a preset amount of time tTA. The hydraulic pressure is then increased in a first upward sweep based on the calculated slope. At a point in time when the hydraulic pressure reaches the target hydraulic pressure PTA, a smaller slope δPTA is set based on a target rotational speed change rate corresponding to when an input rotational speed undergoes a prescribed amount of change. The hydraulic pressure is then increased in a second upward sweep based on the slope δPTA. When the rotational speed change amount ΔN of the input rotation reaches a rotational speed dNS corresponding to an amount of change in the rotational speed that can be detected by an input shaft rotational speed sensor so as to determine that the rotational speed has started to change, the hydraulic pressure then begins being feedback controlled so as to change at a prescribed slope while the change in the input rotational speed is monitored. Additionally, the target hydraulic pressure PTA, the slope δPTA of the second upward sweep portion, and the target shift start time taim of the second upward sweep portion are corrected with a learning compensation control based on measurements of a target shift start time and the rotational speed change rate at the target shift start time.
The technology disclosed in Japanese Laid-Open Patent Publication No. 2000-110929 checks the change in a transmission input torque frequently during shifting of a transmission that is shifted by changing which of a plurality of frictional engaging elements is engaged. By changing the hydraulic pressure (operating fluid pressure) of a closing frictional engaging element and/or an opening (releasing) frictional engaging element to a value corresponding to a post-shift torque (torque obtained after shifting) frequently in response to changes in the input torque of the transmission during shifting, the torque capacity of the frictional engaging element can be prevented from being excessive or insufficient with respect to the changing transmission input torque. As a result, such undesirable occurrences as revving (racing) of the engine, slow shifting, and large torque lapses can be prevented. As shown in FIG. 7 of Japanese Laid-Open Patent Publication No. 2000-110929, a closing hydraulic pressure command value PC increases, and an opening hydraulic pressure command value PO decreases in order to accomplish upshifting by interchanging the connection states of the frictional engaging elements. During this upshifting, if the transmission input torque Ti changes at a time t2, an initial decreased pressure PO1 of the command value PO is changed to a value corresponding to the changed input torque Ti such that the slope at which the command value PO decreases is changed after the time t2. If the input torque Ti becomes equal to or larger than a prescribed value at a time t5, then a corresponding torque phase ramp slope θ5 of the command value PC is calculated and the slope at which the command value PC increases is changed from the normal value θ1 to a steeper value θ5. If the input torque Ti changes at a time t7, then the torque phase ramp slope θ3 of the command value PC is changed to a slope corresponding to the changed input torque Ti. If the input torque Ti changes at a time t10, then the temporarily constant pressure value PC1 of the command value PC and the temporarily constant pressure value PO1 of the command value PO are changed to values corresponding to the changed input torque Ti.
One type of upshift operation is a power-off upshift in which occurs when a vehicle is moving due to inertia after a driver has accelerated by depressing an accelerator pedal and then released the accelerator pedal. An example of this type of power-off upshifting is disclosed in U.S. Pat. No. 5,890,392 (also published as Japanese Laid-Open Patent Publication No. 10-89456). In particular U.S. Pat. No. 5,890,392 discloses a twin-clutch transmission having two transmission input shafts, one transmission output shaft, and two frictional clutches. One of the frictional clutches is provided on each of the transmission input shafts. When a vehicle in which the twin-clutch transmission is installed accelerates from a stop, one of the two clutches is put into a static frictional state from which it transmits engine torque and the other of the two clutches is released.
In U.S. Pat. No. 5,890,392, a case is illustrated in which a higher gear range is selected while a clutch corresponding to a lower gear range is connected and a clutch corresponding to a higher gear range is still in a released state (see FIG. 3 of U.S. Pat. No. 5,890,392—a time chart for explaining the power-off upshift control). The vehicle is traveling in a power-off state, i.e., an inertia state, and, thus, engine is delivering a negative torque. In other words, the engine is operating in an engine braking state. The contact pressure and/or the stroke of the clutch of the low gear range decreases until the clutch enters a slipping state in which the clutch slips slightly. While the clutch of the low gear range is in this slipping frictional state, it still continues to transmit all of the engine torque.
During inertia travel, it can be estimated that the engine is running in an inertia state because the rotational speed of the engine will be lower than the input shaft rotational speed of the lower gear range. The clutch of the lower gear range is controlled first and is controlled such that it is released. When the clutch of the lower gear range is released, the engine changes from a state in which it is delivering negative torque, i.e., is having torque delivered thereto, to a state in which torque is not delivered and, thus, the engine speed declines. Meanwhile, a slip controller of the clutch of the lower gear range operates and adjusts the engine speed to a rotational speed (target rotational speed) just below the rotational speed of the higher gear range.
The slip controller then controls the clutch of the lower gear range such that the engine speed is held just below the rotational speed of the higher gear range while the clutch of the higher gear range is closed in a ramp-like manner (along a sloped path, i.e., a state rate of change). As a result, the slip controller of the lower gear range is released more and more. When the clutch of the lower gear range is completely released, the lower gear range can be disengaged. The clutch of the higher gear range is closed in a ramp-like fashion until it enters a static frictional state. In this way, the power-off upshift is accomplished.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved automatic transmission control apparatus. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.